Reaction injection molding (RIM) has become an important process for the manufacture of a wide variety of moldings. The RIM process is a so-called "one-shot" process which involves the intimate mixing of a polyisocyanate component and an isocyanate-reactive component followed by the injection (generally under high pressure) of the mixture into a mold with subsequent rapid curing. The polyisocyanate component is generally a liquid isocyanate. The isocyanate-reactive component generally contains a high molecular weight isocyanate-reactive component (generally a polyol), and usually contains a chain extender or cross-linker containing amine or hydroxyl groups. U.S. Pat. No. 4,218,543 describes one particularly commercially significant RIM system, which requires the use of a specific type of aromatic amine as a cross-linker/chain extender. The preferred amine described in the '543 patent is diethyl toluene diamine (DETDA). Formulations based on DETDA are generally restricted to the lower flexural modulus range (i.e., less than about 70,000 psi at room temperatue). While it is known to use DETDA in combination with other co-chain extenders in order to increase the flexural modulus of the resultant molding, the use of such co-chain extender generally adversely affects the thermal properties of the resultant part.
In general, the art has looked to various techniques for enhancing the flexural modulus of a RIM part, including use of different aromatic amines (see, e.g. U.S. Pat. No. 4,442,235), use of an aliphatic amine co-chain extender (see, e.g. U.S. Pat. No. 4,269,945), and the use of reinforcements such as fibers, particulate fillers and flakes (see, e.g., Journal of Cellular Plastics, September/October 1981, pages 268-273).
Another technique for enhancing the flexural modulus of RIM parts is to use a reinforcing fiber mat. Problems have been seen in using such mats in the RIM process including displacement of the mat in the mold. incomplete filling of the mold, and part distortion. It is believed that these problems are caused in part by the RIM reactants reaching a high viscosity in too short a time to completely impregnate the mat. One solution to this problem was described in U.S. Pat. No. 4,435,349. The '349 patent describes the use of a reaction mixture of a polyisocyanate, a polyol having an equivalent eeight above 500, a relatively low molecular weight chain extender (such as ethylene glycol), and a delayed action catalyst. In all the examples of the '349 patent, an excess of a polyol was used which had an equivalent weight in excess of 1800 and a molecular weight of no less than about 5500.
U.S. Pat. No. 4,065,410 describes a RIM process wherein the reaction mixture comprises a polyisocyanate, a polyol having a molecular weight of from 1800 to 10,000, a blowing agent, and a chain extender mixture comprising ethylene glycol and another polyol having a molecular weight below 1800. As disclosed in the '410 patent the chain extender mixture comprises from 10 to 30% by weight based on the weight of the high molecular polyol. A similar system is described in U.S. Pat. No. 4,341,875.